Potentiostat circuits are well known for biasing and controlling amperometric electrochemical cells. Such circuits are shown in FIG. 4 of U.S. Pat. No. 4,169,779, FIG. 4 of U.S. Pat. No. 4,171,253 and FIG. 3 of U.S. Pat. No. 4,326,927. These circuits are used in connection with a standard electrochemical cell or sensor which typically has three electrodes: (1) a sensing or working electrode; (2) a counter electrode; and (3) a reference electrode. The sensing electrode is used to detect the presence of the subject gas such as carbon monoxide (CO) or hydrogen sulfide (H.sub.2 S). The counter electrode and the reference electrode are usually connected to the potentiostat circuit which controls the operation of the electrochemical cell. There is, however, no direct electrical connection, and therefore essentially no current flows between the reference electrode and the sensing electrode.
U.S. Pat. No. 4,776,203 describes a potentiostat circuit wherein there is an electrical connection between the sensing electrode and the working electrode when the electrochemical cell is not operating. This connection, however, is eliminated when the electrochemical cell is operating. When the gas monitor containing the electrochemical cell is not being used, the sensing and reference electrodes are customarily connected together in a short circuit, or optionally via a resistor, in order to ensure that the monitor will produce reliable readings quickly after it has been started. If the sensing and reference electrodes are not initially connected, a large start-up current will be observed during initial operation. This patent teaches that it is necessary to break this electrical connection when the monitor is started up and while it is operational. The making and breaking of this connection is accomplished through a "Field Effect Trasistor (FET)" which has a very small resistance when it is turned off (effectively acting like a short circuit) and a very high resistance (effectively acting like an open circuit) when it is turned on. Thus, during operation, there is no direct electrical connection between the sensing electrode and the reference electrode.
From these references, it is clear that the sensing electrode of a three-electrode electrochemical cell used in a gas monitor can be controlled by a potentiostat circuit over a wide range of potentials. A special case exists for those gas monitors whose potentiostats are set to control the sensing electrode at substantially the same potential as the reference electrode, i.e., at a potential of essentially zero millivolts with respect to the reference electrode. Electrochemical cells which operate with a zero-millivolt potentiostat ideally require no current from the potentiostat circuit when the gas to be detected is absent. This occurs only if the potential of the sensing and reference electrodes are nearly identical when no current is drawn from the cell, i.e., when the rest potential is zero. Any deviation from this ideal case results in an undesirable zero-gas current when no gas is present. Although this current may be compensated for with an additional electronic compensation circuit, it would be simpler and less expensive if such compensation were not required.
It would be desirable therefore to have a potentiostat circuit which not only eliminated the start-up current but also significantly reduced the zero-gas current without having to make or break an electrical connection between the sensing electrode and the reference electrode.